I am wondering if preheating the transaxle will improve fuel efficiency. If so then what temperature should the preheat reach and what wattage heater is to be used assuming the heating time is 2 hours? Thanks!
The transaxle? I suppose in the coldest winter it might benefit by reducing the lubricant viscosity a little, but the energy used for that could be better applied several places. I use an engine block heater (EBH) on the engine (ICE) which helps reduce the warm up time dramatically. There's potential benefit of prewarming the battery--some have speculated that using a waterbed heater on the traction battery case in conjuction with the EBH would help a lot in very cold climates b/c if the battery gets very cold it loses a LOT of efficiency. The transaxle is not where I would focus.
Thanks for the comments! I have been using EBH for a little while so I am thinking about other possible improvements.
Well, heating the transaxle would reduce the first few rough ICE off transitions. Bob Wilson did some tests on the GI to see what the effects of heat would be. Perhaps he will answer for you.
I'm no engineer, but from what I understand of the Prius HSD transaxle, you don't want to heat that fluid. The two electric motor/generators in the HSD drive is immersed in the transaxle fluid. You heat that stuff up, it will increase the resistance in the windings of the electric motors. That's actually BAD for both the longevity of the electric motors as well as efficiency. The Prius tranaxle is NOT a hydraulic fluid coupling like a regular automatic transmission's torque converter (which needs to be heated to a proper operating temperature).
I looked up the temperature coefficient of copper to be 0.4%/deg C. So if we heat the motors from 20C to 50C then the resistance will increase 30 * 0.4% = 12%. Not huge but not negligible either. Now we need to weigh this increase in resistance with the decrease in viscosity.
No offense, but I think any time, effort, expense or energy put toward specifically pre-warming the transaxle is a waste. Via conduction and convection the transaxle will be somewhat prewarmed already since you're using an EBH. Further, it will quickly warm up as the engine compartment warms up under normal operation. I'd love to see some numbers from someone using a waterbed heater for the traction battery. Not sure where you live, but when it gets cold (below freezing anyway) the traction battery noticeably loses function/capacity and it can take forever (relatively) to warm the very dense pack up. I've driven when the pack had a chance to drop to 20F ambient temp then drove 3.5 hours home...it was 2.5-3 hours into the drive before the pack reached 50F (I can monitor with CAN-View) and started behaving somewhat normally. The ability to preheat to somewhere around 70F along with the EBH would, I think, allow near normal hybrid performance and efficiency.
Hey no problem at all - I really appreciate everyone who spends his/her precious time helping out in the forum.
There is one documented case of an MG2 overheating failure that Usbseawolf2000 linked to previously... Check it out: Rebuilding a Toyota Prius Transaxle -- Art's Automotive, Berkeley, CA It's in a gen-1 Prius and the MG2 overheating failure was due to a gasket failure on the coolant pump circuit, but I think it demonstrates why putting an extra heat load on the MGs is not a good idea.
External fluid heat has little effect on the resistance of the windings. Heating up the fluid does not change the resistance of the wires enough to matter. Its not the fluid temp that kills the wires. Its motor load. Pulling more power than the wire/winding can handle. Thats current load capacity. Which is exactly what happened in the story you quoted. Loss of lubrication made bearings fail making the motor pull much more electric load the windings were designed to provide. Winding blowout. The fluids job is not winding cooling its bearing lubrication. Make the motor run harder than designed and it will blow out the windings. No amount of fluid on board will fix that. The inverter is very tough. It will provide enough power to do it. Besides your scientific notation shows that more heat increases resistance. That means less power delivered to the motor across the windings ( less current draw across the motor) and making the inverter work harder to supply the same power to the motor. If you want better conductivity through your windings and deliver more power with less effort you have to lower the temp. Colder it is the less resistance if your talking windings. Put the best fluid in the cvt you can buy. Keep the cvt operating temp down by smooth bearing operation.
